(a) Field of the invention:
This invention relates to a power amplifier circuit adapted for use in stereo sets, etc. and more particularly to a final stage current amplifier capable of changing the operation mode between class B (or AB) and class A.
(b) Description of the Prior art:
The operational (bias) mode of an audio amplifier is usually class A or class B (or AB). The class A amplifier has an advantage that the distortion is small but a disadvantage that the efficiency is low, while the class B amplifier has the advantage and the disadvantage of the opposite nature. When a higher efficiency is desired, as is usually the case for a power amplifier, a class B push-pull circuit is usually adopted. In the class B push-pull circuit, however, distortion inevitably exists to some extent in the output signal. Such distortion becomes especially noticeable in the small signal range due to the basic principle of the class B amplifier. However, if the mode of bias can be changed over between class B and class A, a wider range of demands can be satisfied. Namely, a power amplifier can work in class B mode when a higher output power is demanded, and also in class A mode when the output signal of higher quality is demanded even if the maximum output power is reduced.
Examples of such mode-changeable power amplifiers are proposed in Japanese Patent Laid-Open Application Nos. 49-91567 and 49-91568. In the power amplifier of Japanese Patent Laid-Open Application No. 49-91567, a switch for changing over the bias voltage is provided in the circuit for setting the bias voltage for the amplifying transistor of the final stage current amplifier circuit. The bias point of the final stage current amplifier is changed by this change-over switch, thereby changing the mode of bias. According to this structure, however, the length of the wiring between the bias change-over switch and the bias setting circuit becomes long and thereby the output signal is easily influenced by the external noises. Further, the wiring operation may become complicated. Yet further, since the bias change-over switch is operated externally and has a mechanical contact, there may arise poor electrical connection at the contact.
In the power amplifier circuit of Japanese Patent Laid-Open Application No. 49-91568, a bias change-over switch is provided in the bias setting circuit for the amplifying transistor to change-over the bias voltage and hence the operation mode (class) of the final stage amplifier circuit, similar to the amplifier circuit of Japanese Patent Laid-Open Application No. 49-91567, and further a switch interlocked with the bias change-over switch is provided for changing over the supply voltage for the final stage current amplifier circuit.
Generally, in a class A amplifier, the bias point is set at a middle point of the load line, and hence a relatively large idling current is allowed to flow through the transistor. Therefore, there is a need for reducing the supply voltage for the final stage amplifier in the class A operation to a fraction of that in the class B operation. Considering this point, a switch for changing over the supply voltage is provided in this circuit.
Interlocking the bias and supply voltage change over switches, however, cannot completely prevent from flowing in the amplifying transistor in the final stage. Such excess current may cause damage or deterioration of the characteristic of the transistor. More specifically, a considerably large capacitance is usually connected to the supply voltage line and/or the power source circuit. Thus, even when the supply voltage change-over switch is changed over, the supply voltage itself does not change rapidly. When the operation mode of the final stage amplifier is changed over from class B to class A, the biasing voltage may be changed over to a high level in accordance with the class A operation before the supply voltage decreases to a required value. An excess idling current thus flows to possibly damage the amplifying transistor of the final stage current amplifier circuit. To avoid such disadvantages, a time lag may be given to the actuation time of the bias change-over switch and that of the supply voltage change-over switch. This, however, leads to a complicated structure. Further, since the rising or falling time of the supply voltage may vary over in a wide range depending on the operation timing and the successive operations, it is not easy to realize a switch satisfying these conditions. Further, drawbacks met in the foregoing example still exist in this example.